1. Field of the Invention
This invention relates to a projection apparatus for projecting the image of a light valve onto a screen. Particularly, the present invention is suitable for a video projector.
2. Related Background Art
In recent years, so-called video projectors are known as apparatuses for projecting images onto a relatively large screen. These video projectors are divided broadly into one type comprising a construction of the phase modulation type in which the birefringence of liquid crystal and a polarizing optical system are combined together and one type comprising a construction of the scattering type in which liquid crystal having a light scattering property like polymer dispersion type liquid crystal and a schlieren optical system are combined together.
In such a video projector, it is common to apply a beam of illuminating light from a light source onto the image information display surface of a light valve and apply a beam of spatially modulated light onto a screen, which is a projection thereby projecting an image. It is required that the image be bright when the beam of illuminating light is applied onto the light valve with high illuminance (intensity) and is projected onto the screen.
Generally, to apply a beam of illuminating light onto a light valve with relatively high illuminance (intensity), the latter video projector of the scattering type is more advantageous than the former video projector of the phase modulation type in as much as it does not use polarized light and suffers little from the loss of the quantity of light. Various video projectors of this type have heretofore been proposed.
The video projectors of this scattering type include one type having a construction in which light regularly reflected by the image information display surface of a light valve is projected onto a screen and one type having a construction in which light scattered by the image information display surface of a light valve is projected onto a screen. The former is more convenient to obtain a bright image.
In such video projectors, use is generally made of white light sources emitting a beam of illuminating light of high brightness such as metal halide lamps and xenon lamps. A combination of these white light sources and, for example, a concave mirror such as an elliptical mirror, a parabolic mirror or a spherical mirror and a condenser lens or the like is generally used as light source means or an illuminating optical system.
Specific examples of the scattering type video projector according to the prior art are shown in FIGS. 1 and 2 of the accompanying drawings. Both of the scattering type video projectors shown in FIGS. 1 and 2 are of a type which utilizes regularly reflected light.
In FIG. 1, a beam of illuminating light emitted from a light source 71 is reflected and condensed by an elliptical mirror 72 and is substantially collimated by a condenser lens 73, whereafter it is applied so as to have any angle with respect to the image information display surface of a light valve 76. On the light valve 76, the applied beam of light is reflected and directed to a projection lens 75, and, at this time, this reflected beam of light is spatially modulated in conformity with image information.
The projection lens 75 causes the beam of light regularly reflected on the light valve 76 to be imaged on a screen 78. A stop 79 is provided at a location whereat the beam of regularly reflected light is condensed by the projection lens 75. Light scattered by the image information display surface of the light valve 76 is eliminated as an unnecessary beam of light by the stop 79.
As described above, the beam of illuminating light applied to the light valve 76 by an illuminating optical system comprising the light source 71, the elliptical mirror 72, the condenser lens 73, etc. is spatially modulated in conformity with image information displayed on the image information display surface of the light valve 76, and thereafter is directed to a projection optical system comprising the projection lens 75, etc. and is projected onto the screen 78.
Such a projection apparatus of the scattering type must be designed such that the illuminating optical system and the projection optical system do not interfere with each other. Therefore, like the example of the prior art shown in FIG. 1, it is made into such a construction that the optical axes of the two optical systems are spatially inclined with respect to the image information display surface of the light valve. This has resulted in the problem that the image projected onto the screen is distorted.
In order to reduce such distortion of the image, there has also been proposed a projection apparatus in which respective members are disposed so as not to bend the optical path of a beam of light from image information to the utmost. FIG. 2 shows an example of a projection apparatus of such a construction.
Light emitted from a light source 81 is reflected and condensed by an elliptical mirror 82 and is reflected toward a light valve 86 by a mirror 84. A condensing lens 85 is provided between the mirror 84 and the light valve 86. The beam of light reflected by the mirror 84 is substantially collimated by this condensing lens 85, add thereafter enters the image information display surface of the light valve 86 at an angle nearly at a right angle. The beam of light which has entered the light valve 86 is reflected on the image information display surface of the light valve 86 and again enters the condensing lens 85, but, at this time, this reflected beam of light has been spatially modulated in conformity with image information.
The beam of light which has emerged from the condensing lens 85 enters a projection lens 87 through a stop 89. This stop 89 is for preventing the light scattered by the image information display surface of the light valve 86 from arriving at the projection lens 87. After scattered reflected light is eliminated as an unnecessary beam of light by this stop 89, an image is formed on a screen 88 by the projection lens 87.
However, even if, as shown in FIG. 2, the respective members are disposed so that the beam of illuminating light may enter the light valve at an angle approximate to a right angle, thereby causing the beam of illuminating light to enter at an angle approximate to a right angle, it is impossible to cause the beam of illuminating light to enter exactly at a right angle. Therefore it is unavoidable for the image to be distorted to a certain degree. Further, in the case of such a construction, the performance requirements of the projection optical system become very strict, and this gives rise to another problem that not only design is difficult, but also manufacture takes much time, which leads to an increase in cost.
It is also conceivable to provide a half mirror in the optical paths of the illuminating light and projected light, bend the optical path of the projected light or the reflected beam of light by 90.degree., cause the incident light to enter the light valve exactly perpendicularly thereto, and also cause the beam of spatially modulated light to be reflected in a perpendicular direction and projected onto a screen (parallel to the surface of the light valve). An example of such construction is shown in FIG. 3 of the accompanying drawings.
FIG. 3 shows a projection apparatus of such a construction that a beam of illuminating light is caused to enter a light valve 96 perpendicularly thereto. Between a condenser lens 93 and light valve means, a half mirror 94 is provided so as to keep an inclination of 45.degree. with respect to the light valve means. The projector is designed such that one of the beams of light is reflected by the half mirror 94, whereby the optical path of the beam of illuminating light and the optical path of the reflected beam of light differ from each other.
Light emitted from a light source 91 is reflected and condensed by an elliptical mirror 92 and is substantially collimated by a condenser lens 93, and thereafter enters the half mirror 94. The half mirror 94 reflects part of the incident light but transmits part of the incident light therethrough. Thus, about a half of the quantity of light of the beam of light emitted from the flight source 91 arrives at the light valve 96.
The reflected beam of light reflected by the light valve 96 and spatially modulated at the same time enters the half mirror 94 again, and is half-transmitted (half-reflected) therethrough. Only the reflected beam of light is directed to a condensing lens 95 through a stop 99. Accordingly, the quantity of light finally arriving at a projection optical system becomes about a quarter of the quantity of light emitted from the light source 91, which means a considerable loss of the quantity of light, and this has led to the problem that an image projected onto a screen generally becomes dark.